Fourth generation (4G) cellular networks include a radio access network (e.g., a long term evolution (LTE) network or an enhanced high rate packet data (eHRPD) network) and a wireless core network (e.g., referred to as an evolved packet core (EPC) network). The LTE network is often called an evolved universal terrestrial radio access network (E-UTRAN). The EPC network is an all-Internet protocol (IP) packet-switched core network that supports high-speed wireless and wireline broadband access technologies. An evolved packet system (EPS) is defined to include both the LTE (or eHRPD) and EPC networks. EPS improves mobile technology by providing higher bandwidth, better spectrum efficiency, wider coverage, enhanced security, and full interworking with other access networks. EPS proposes provides these improvements using an all-IP architecture.
A packet data network (PDN) gateway (PGW) is one network element provided in the EPC network. When user equipment (UE) connects or attaches to the EPC network, the UE is attached or anchored at the PGW in order to provide the UE with access to one or more PDNs. The PGW is the common anchor point for UEs in LTE or eHRPD service areas.
During UE attachment to a PGW or a particular PDN, the EPC network separates the signaling plane and the user plane as the UE communicates with the EPC network. In the EPC network, the signaling plane for LTE access is handled by a mobility management entity (MME), and the user plane for LTE access is handled by a serving gateway (SGW). A UE first communicates with a MME during a UE authentication and authorization process. During this process, the MME selects a PGW for the UE (e.g., when the UE attempts to connect to a particular PDN). The selection of a PGW for a particular PDN may be a static process or a dynamic process. When the UE signals a MME for a PDN connection, the MME interacts with a home subscriber server (HSS) in order to authenticate and/or authorize the UE. The HSS provides the MME with UE (or subscriber) profile data that includes a static or dynamic field (e.g., a PGW allocation type field). When the field is static, the HSS provides the MME with a fully qualified domain name (FQDN) of the PGW to be used by the UE to access the PDN. However, when the field is dynamic, the MME forms the FQDN by selecting a geographically closest PGW to the UE.
The signaling plane of an eHRPD network is handled by a HRPD serving gateway (HSGW). Like the MME, the HSGW aids the UE by forming a conduit for UE authentication and/or authorization with an authentication, authorization, and accounting (AAA) device (e.g., which accesses the same subscriber database as the HSS). The HSGW also utilizes the same PGW allocation type field, which is set as static or dynamic for PGW selection. For the static case, the AAA device returns the PGW FQDN, and for the dynamic case, the HSGW forms the PGW FQDN by selecting a geographically closest PGW to the UE.
A UE sets up each PDN connection via MME selection of a PGW for each PDN connection request. The PGW selected for different PDN connection requests may be the same physical PGW or it may be different PGWs. When the MME selects a PGW for a PDN connection request, the MME sends a query to a domain name system (DNS) server for IP address resolution of the selected PGW's FQDN. The DNS server contains IP address mappings to PGW FQDNs. The DNS server provides a response (e.g., to the MME query) that includes a single IP address (e.g., of the selected PGW) corresponding to the selected PGW's FQDN. However, the MME cannot determine the availability of the selected PGW since no direct signaling exists between the MME and PGWs. If the selected PGW is unavailable (e.g., because a user plane link between the SGW and PGW is down, because the PGW is out of capacity and cannot serve any more PDN sessions, etc.), the PDN connection request will fail and the UE will have to establish a PDN connection all over again.
To maintain service availability, two or more PGWs may be configured in an active/standby pair. This may be accomplished by enabling Inter-Chassis Session Recovery (ICSR) (e.g., between the PGWs) that supports either local redundancy (e.g., co-located PGWs) or geographical redundancy (e.g., PGWs with different locations). The PGWs that participate in one logical ICSR group share a single virtual IP (VIP) address. For example, an active PGW located in San Francisco and a standby PGW located in Los Angeles may be configured as an ICSR group. The two PGWs may be assigned a single VIP address (e.g., “0.0.0.1”). If the active PGW in San Francisco fails, the standby PGW in Los Angeles may automatically become active. Since both PGWs share the same VIP address (e.g., “0.0.0.1”), none of the underlying network elements would notice the active PGW's failure.
However, in such an arrangement, the MME only supports a single VIP address for PGW selection. Furthermore, even though ICSR provides inter-chassis redundancy across more than one PGW, certain catastrophic failures which can cause total network outages (e.g., IP routing failures, line cuts, loss of power, physical destruction of the chassis participating in the ICSR, etc.) cannot be protected against by this arrangement. In such cases, the single VIP address stored by the MME will become unreachable. This may result in a total or a major network outage.